The present invention relates to clock signal generating circuitry for communications systems and, more particularly, to redundant clock signal generating circuitry for reliably providing an uninterrupted clock signal.
In time-division-multiplex (TDM) communication systems, it is necessary to have a continuous and uninterrupted clock signal in order to properly receive and transmit digitized speech signals multiplexed on incoming and outgoing highways. In the prior art, clock generating circuits have utilized two oscillators, such as those described in U.S. Pat. Nos. 3,628,158 and 3,795,872; three oscillators, such as that described in U.S. Pat. No. 3,662,277; and even four oscillators, such as that described in U.S. Pat. No. 3,769,607, in efforts to more reliably provide an uninterrupted clock signal.
Switching between the several clock signals may be based on any of several criteria, such as the loss of one signal, phase slippage between signals, and frequency differences such as that reflected by changes in the pulse widths of the signals. However, when switching between clock signals, it is extremely important that the switching be transparent to circuitry utilizing the clock signals. Thus, each of the clock signals must be phase and frequency coincident with one another, otherwise switching may cause phase discontinuities and momentary bit transitions. Phase-locked loop techniques have been utilized to some degree to provide for transparent switching. In clock generators utilizing phase-locked loop oscillators, a malfunction typically causes both oscillators to free run, thus, no longer maintaining phase and frequency coincidence between the clock signals provided thereby. Switching in response to any further malfunctions would then introduce the possibility of phase discontinuities and momentary bit transitions.
Accordingly, it is an object of the present invention to provide an improved clock signal generator for providing redundant clock signals that are both phase and frequency coincident with one another.
It is another object of the present invention to provide an improved clock signal generator for providing redundant clock signals that are continuously phase and frequency coincident with one another, regardless of which is designated the master clock signal.
It is yet another object of the present invention to provide an improved clock signal generator for providing redundant clock signals that also provides for improved diagnosis of malfunctions therein.
It is yet a further object of the present invention to provide an improved clock signal generator for providing redundant clock signals, wherein designation of the master clock signal may be externally controllable.